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C H A P T E R 7 Latent Print Development 7.11 Vacuum Metal Deposition 7.11.1 History Vacuum metal deposition (VMD) is a long-established industrial technique for the application of metal coatings to components such as glass to form a mirror. In 1964, phys- ics professor Samuel Tolansky (Royal Holloway College, University of London) noted that the deposition of silver in a vacuum system developed latent fingerprints accidentally deposited on a glass component. An investigation into the process as a fingerprint development technique was pro- posed. However, this was not pursued at the time by the U.K. Home Office because other techniques for fingerprint detection on glass were considered cheaper, easier to use, and sufficiently effective. In 1968, French workers reported (Theys et al., 1968, p 106) that VMD of a mixture of zinc, antimony, and copper powder was capable of developing latent prints on paper. As a consequence of this article, interest in the technique was revived in the United Kingdom, and Tolansky initiated a research program to investigate the optimum conditions and the potential applications for VMD. One of the early objectives of the research was to establish why the French composition was effective. Closer examination of deposited metal coatings produced by the French laboratory indicated that the coating was almost entirely zinc, the presence of antimony and copper not being necessary to develop prints (Hambley, 1972). The research program initiated by Tolansky (Hambley, 1972) investigated the deposition characteristics of a range of metals on paper substrates, identifying single metals and metal combinations giving the optimum print development. Research was also conducted into the ability of the technique to detect latent prints on fabrics. These experiments showed that although some print development was obtained by the use of single metals (e.g., gold, silver, copper, zinc, and cadmium), in general, the best results were obtained by the use of a combination of metals, typi- cally gold or silver followed by cadmium or zinc. Initially, the gold and cadmium combination was selected as the optimum, although subsequent health and safety issues have resulted in the gold and zinc combination being recommended instead. Gold was preferred over silver as the initial deposition metal because silver can be degraded by fingerprint secretions and atmospheric pollutants. 7–34 The early experimental work was carried out on small- scale equipment with a bell jar, but research continued to develop larger equipment suitable for use in a fingerprint laboratory. By the mid-1970s, systems modified from standard industrial equipment had been developed (Kent, 1982) and were in use in several police forces and forensic providers within the United Kingdom. Later, manufacturers made refinements, increasing the size of the vacuum chamber and adding controls specific to the fingerprint development process. In the 1990s, the technique made its way from Europe to North America (Murphy, 1991, pp 318–320; Misner, 1992, pp 26–33; Masters and DeHaan, 1996, pp 32–45). Specially constructed VMD equipment is now supplied by several manufacturers worldwide. Although VMD was originally investigated as a fingerprint development technique for use on paper and fabrics, it was established that other processes are capable of giving bet- ter results on paper. However, VMD was found to give ex- cellent results on nonporous substrates and in comparative studies was found to outperform all other techniques in developing marks on plastic bags (Misner, 1992, pp 26–33; Kent et al., 1975, 1978; Reynoldson and Reed, 1979). The process was also found to develop marks on substrates exposed to water and conditions of high humidity, giving substantial advantages over techniques such as CA fuming for articles that have been exposed to these conditions. Few modifications have been made to the process itself since the change in the second deposition metal from cadmium to zinc in the late 1970s. Recently, there has been further research on VMD in Australia, looking in detail at the various print development regimes that can be followed on different grades of polyethylene (Jones et al., 2001c, pp 73–88) and how the surfaces could be “reac- tivated” to develop prints if excess metal deposition had occurred initially (Jones et al., 2001d, pp 5–12). The work was extended to investigate other polymer substrates, including polypropylene, polyvinylchloride, and polyethylene terephthalate (Jones et al., 2001b, pp 167–177), and different deposition conditions were recommended for each class of polymer, in particular the amount of gold deposited prior to zinc deposition (polyethylene terephthalate and polyvinylchloride require significantly more gold to develop prints than polymer or polypropylene). However, there are situations where the performance of VMD leaves much to be desired. It is believed that the effectiveness of VMD can be detrimentally affected by the
presence of body fluids (Batey et al., 1998, pp 165–175) and drug residues (Magora et al., 2002, pp 159–165). It has also been difficult to develop prints on heavily plasticized polymers (such as clingfilm and plasticized vinyl) using the VMD process. Recent work has indicated that deposition of silver as a single metal may give improved detection rates over the gold and zinc combination for these types of substrates, and the silver deposition process has now been published for operational use (Home Office Scientific Development Branch, 2005, pp 8–9). 7.11.2 Theory There is general agreement on the theory associated with normal development of prints by the VMD method. The reason that the metal combinations are postulated to work well is due to the condensation characteristics of zinc (and cadmium). These metals will not condense on grease, such as that found in fingerprint residues, even when the oily residues are present only as a monolayer. However, zinc will deposit on small nuclei of metal, and this is the reason that gold or silver deposition is carried out first. Gold and silver can be deposited over the entire surface and begin to form nuclei, the morphology of which depends on the nature of the surface (surface energy, chemical species present) upon which they are being deposited. The resultant gold coating is very thin (several nanometers only) and discontinuous. However, in the regions coated with the fatty residues of the latent fingerprint, the gold diffuses into the fat and hence there are no gold nuclei close to the surface. As a consequence, when zinc is subsequently deposited, it will condense on the regions of gold nuclei (i.e., the background substrate) but not on the regions of the fatty deposit (i.e., the fingerprint ridges). This theory of nucleation was discussed in more detail by Stroud (1971, 1972). The normal development process is depicted schematically in Figure 7–18, and a photograph of a mark produced by normal development is shown in Figure 7–19. Latent Print Development C H A P T E R 7 FIGURE 7–18 Schematic diagram of normal development, showing zinc depositing where gold nuclei are available on the surface. Tests carried out to determine which components of the latent print were most likely to be responsible for inhibiting metal deposition identified several substances, including stearic acid, palmitic acid, cholesterol oleate, glycerol trioleate, and amino acids L - arginine monohydrochloride, L - leucine, and DL - threonine. Most of these substances are non-water- soluble or long-chain fats or acids with low vapor pressure, which determines their stability and nonmigration over the surface during the VMD process. These findings were in accord with the observation that VMD was capable of developing prints on substrates exposed to water. Experiments to study the diffusion of gold into thin films of stearic acid (Thomas, 1978, pp 722–730) demonstrated that 60% of the gold penetrated the stearic acid to a depth greater than the detection depth of the electron spectroscopy for the chemical analysis (ESCA) surface analysis technique and hence would probably not be sufficiently close to the surface for zinc to nucleate on it. Electron microscopy has also been used to confirm that the size and distribution of gold nuclei formed during the deposition process varied greatly according to the substrate and the chemical species present (Kent, 1981, p 15). It was this difference in nuclei size and distribution, coupled with diffusion of gold into the fatty deposits, that subsequently delineated the print during VMD. In practice, many prints developed using VMD may be “reverse developed” (i.e., zinc preferentially deposits on the fingerprint ridges rather than the background). There are differences in opinion as to why this arises (Jones et al., 2001b, pp 167–177; 2001c, 73–78; Kent et al., 1976, p 93), but none of the theories have been categorically proven, and in some cases reverse and normal development may be observed on the same substrate (although it is stated that this is most common for [if not exclusive to] low-density polyethylene substrates). Figure 7–20 shows a reverse-developed mark on a polyethylene bag. 7–35
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